Adiponitrile is an important starting material in nylon production, which is obtained by double hydrocyanation of 1,3-butadiene. In a first hydrocyanation, 1,3-butadiene is hydrocyanated to 3-pentenenitrile, while the by-products obtained are mainly 2-methyl-3-butenenitrile, 4-pentenenitrile, 2-pentenenitriles, 2-methyl-2-butenenitriles, C9 nitriles and methylglutaronitrile. In a second, subsequent hydrocyanation, 3-pentenenitrile is reacted with hydrogen cyanide to give adiponitrile. Both hydrocyanations are catalyzed by nickel(0)-phosphorus complexes.
A general review of nickel-catalyzed olefin hydrocyanation is described in Tolman et al., Adv. Cat. 33, 1-46 (1985).
The hydrocyanation of 1,3-butadiene using a nickel catalyst of the formula Ni[P(OR)3]4 is described in U.S. Pat. No. 3,496,215. A disadvantage of this process is that no suitable technique is specified for the complete recovery of the 1,3-butadiene or of the catalyst.
The performance of the hydrocyanation in one or more reactors and their connection is described in U.S. Pat. No. 4,810,815, and mention is made of the possibility of continuous operation of stirred tanks or batteries of stirred tanks, but only a semibatchwise mode is described in detail in examples, from which the person skilled in the art cannot discern directly the conditions under which the method should proceed in continuous stirred tanks.
A process for removing organic phosphorus compounds and their metal complexes from organic nitriles in the hydrocyanation of olefins is described in DE 10 2004 004671. The removal is effected by contacting the product with a cycloparaffin or paraffinic hydrocarbon. This forms a liquid multiphase system.
1,3-Butadiene can polymerize. Therefore, stabilizers, for example tert-butylpyrocatechol (TBC), are added to the 1,3-butadiene.
The nickel(0) complexes used for the hydrocyanation of 1,3-butadiene are water-sensitive. Water-comprising 1,3-butadiene therefore has to be dried before the hydrocyanation.
DE-A-10 2004 04684 discloses the drying of 1,3-butadiene comprising water and a stabilizer with the aid of microporous solids. This adsorbs not only the water, but also the stabilizer, at least partly from the solids. Suitable microporous solids are aluminum oxide and molecular sieves.
Heterogeneous dessicants, as a result of the adsorption of stabilizers, have the disadvantage that, at least temporarily, 1,3-butadiene which is insufficiently stabilized, if at all, forms, and is subject to safety restrictions. For instance, unstabilized 1,3-butadiene can be used only at temperatures below 15° C. In addition, a further disadvantage is that the microporous adsorbents have to be regenerated regularly.
In addition, it is crucial for an integrated process for preparing 3-pentenenitrile, in which both 1,3-butadiene and the hydrocyanation catalyst stream are recycled, that the 1,3-butadiene used in a molar excess relative to hydrogen cyanide is recycled efficiently.
DE-A-10 2004 04724 discloses the reaction of stabilized 1,3-butadiene, which has been dried with the aid of microporous solids, with hydrocyanic acid in the presence of nickel(0) catalysts to give 3-pentenenitrile. Three distillation columns serve for the removal and recycling of unconverted 1,3-butadiene and of nickel(0) catalyst.